1. Technical Field
The present invention relates to an optical integrated circuit and a manufacturing method thereof, and, more particularly, to an optical integrated circuit which couples light from an optical waveguide to a photodetector with a low loss and a method of manufacturing the structure.
2. Related Art
A conventional waveguide optical signal detector comprises an SiO2 buffer layer formed on one major surface of, for example, a silicon substrate by thermal oxidation, a photodetector provided by an ordinary semiconductor process, a tapered optical coupling area formed on the photodetector by Local Oxidation of Silicon (LOCOS), and an optical waveguide layer formed thereon.
In a case where the optical waveguide layer is of a phase matching type which takes a multi-layer structure, at least one optical waveguide layer other than the main optical waveguide layer and a cladding layer which is sandwiched between the optical waveguide layers are alternately deposited on the photodetector formed on the substrate.
In a structure or assembly that has a tapered optical coupling area, however, the substrate on which a SiO2 buffer layer can be formed by the thermal oxidation is limited to a silicon substrate. Further, as the structure requires a process to be conducted under a vacuum condition and a high-temperature treatment, the fabrication process is complicated and the structure is expensive.
In a case where the optical coupling area takes a simple tapered structure, it is known that propagating light suffers a large optical loss at the curve portion of the optical waveguide layer. To reduce the light loss, a scheme of controlling the taper angle has been considered, which, however, is difficult to achieve.
When a multi-layer structure is adopted, the process is originally complex. As optical coupling is based on phase matching, an optimal structure is needed for each wavelength of light to be used. Therefore, this structure is not suitable for detection of multi-wavelength signal light having different wavelengths to be detected.
To ensure efficient signal detection, the latter structure requires a relatively large photo-detection area as compared with the former structure that directly leads the waveguide layer to the photo-detection device, and is not therefore suitable for making the whole device, including the photo-detection device, compact.
Accordingly, it is an object of the present invention to provide an optical integrated circuit which reduces a waveguide loss within a waveguide from an optical waveguide to a photodetector.
It is another object of the present invention to provide a method for manufacturing the above optical integrated circuit easily and at a low cost.
According to a first aspect of the invention, there is provided an optical integrated circuit comprising: a photodetector formed on a substrate; a cladding layer formed over said photodetector on one major surface of said substrate, said cladding layer having a first refractive index; and an optical waveguide layer formed on said cladding layer and having a second refractive index greater than said first refractive index. The cladding layer comprises a plurality of sub-cladding layers overlaid on each other in a thickness direction of said substrate, said cladding layer includes a first portion provided over the photodetector, a second portion provided on an area of the photodetector except for the first portion, said second portion having less thickness than that of said first portion, and a smooth curve portion coupling said first portion to said second portion.
According to another aspect of the invention, there is provided a method for manufacturing an optical integrated circuit in a substrate, having a photodetector provided in a surface of the substrate, a light shielding layer positioned on and around a peripheral portion of said photodetector, and an optical waveguide provided over the photodetector and a portion of the light shielding layer, the light shielding layer having a step-portion on and around a periphery portion of the photodetector, comprising the steps of: forming a cladding layer having a plurality of sub-cladding layers overlaid on each other in a thickness direction on said substrate, said cladding layer having a thickness to enable light to be guided to said photodetector; and forming an optical waveguide layer on said cladding layer, said optical waveguide layer having a higher refractive index than that of said cladding layer, and said optical waveguide layer being optically coupled to said photodetector. The step of forming a cladding layer includes the steps of applying and curing an insulating liquid on said substrate to form one of the plurality of sub-cladding layers, and repeating the step of applying and curing a liquid insulator a predetermined number of times, where said sub-cladding layers cover said step-portion.
According to further aspect of the invention, there is provided a method for manufacturing an optical integrated circuit on a substrate, said optical integrated circuit having a photodetector and an optical waveguide provided over the photodetector, comprising the steps of: forming a cladding layer having a plurality of sub-cladding layers overlaid on each other in a thickness direction on said substrate, said cladding layer having a thickness permitting light to be guided to said photodetector; and forming an optical waveguide layer on said cladding layer, said optical waveguide layer having a higher refractive index than that of said cladding layer, and said optical waveguide layer being optically coupled to said photodetector. The step of forming a cladding layer includes the steps of applying and curing an insulating liquid on said substrate to form one of the plurality of sub-cladding layers, and repeating the step of applying and curing a liquid insulator a predetermined number of times, wherein said sub-cladding layers cover said step-portion.